The problems keep piling up for Facebook, and it's unclear how long the internet giant will be able to brush them aside as it barrels toward acquiring its next billion users.

The world's biggest social network has unwittingly allowed groups backed by the Russian government to target users with ads. That's after it took months to acknowledge its outsized role in influencing the U.S. election by allowing the spread of fake news — though before news emerged that it let advertisers target messages to "Jew-haters."

Now Facebook is under siege, facing questions from lawmakers and others seeking to rein in its enormous power. The company has turned over information on the Russia-backed ads to federal authorities investigating Russian interference in the U.S. presidential election. Critics say the company also needs to tell its users how they might have been influenced by outside meddlers.

Speculation is rife that Facebook executives, perhaps including CEO Mark Zuckerberg, could be called to testify before Congress. Hearings might lead to new regulations on the company.

"Facebook appears to have been used as an accomplice in a foreign government's effort to undermine democratic self-governance in the United States," writes Trevor Potter, former chairman of the Federal Election Commission and now head of a nonpartisan election-law group, in a letter to Zuckerberg.

Potter's group, the Campaign Legal Center, wants Facebook to make the Russian-sponsored ads public. The company has so far declined to do so, citing the ongoing investigations. It has provided the ads and other information to Robert Mueller, the special counsel in charge of the Russia investigation, Facebook said in a statement, although it declined to elaborate.

The company that nudges its users to reveal intimate details about their lives, it turns out, isn't all that comfortable doing the same. That's true for everything from the secret algorithms that recommend "people you might know" to data on its attempts to clamp down on the spread of false news shared across its network.

The company justifies its secrecy in many ways, having variously claimed legal restrictions, business secrets, security and privacy protections to excuse its opacity. But Jonathan Albright, whose late 2016 research on the "fake news" propaganda ecosystem outlined how propaganda websites track and target users, thinks the current moment may be a turning point for online giants like Facebook.

"Now that it has run directly into something that possibly affected the outcome of the election — but they can't determine how — this may be their era of accountability," said Albright, the director of research at the Tow Center for Digital Journalism at Columbia University.

There has been no other company on the planet, Albright added, that can provide access to as many real people as Facebook.